Then run the last function in the script 'callist()'* to get a list of all your calendars. Go to the first sheet (master) and delete the calendar names ans ID that you don't want to monitor -DON'T LEAVE ANY EMPTY ROW IN THE LIST !

* note that the test calendar that is on the model spreadsheet is really a test Calendar, nothing personal in there ! you can use it for testing purposes if you want

And that's it, you're ready to run it for the first time.

It will ask for authorization on Calendars, spreadsheet and Mail, that is normal . The first email you will receive will be empty since it takes data from the script properties (to 'see' events that has been removed) and script properties is empty on startup.

The last thing you have to do is to setup a trigger for this to work automatically : from the script editor goto ressources > current script triggers and follow the example below.

Now the script will monitor every change on your chosen calendar and send you an email if someone 'not authorized' adds or removes or changes any event... it will check on every hours but you can change that to a shorter time if you want (be careful though not to explode your quota or max execution time). Do not use trigger periodicity of more that 115 minutes because the script uses a trick to detect the 'natural end' of events without generating alerts.